c++: Make braced-init-list as template arg work with aggr init [PR95369]

c++: Make braced-init-list as template arg work with aggr init [PR95369]

Barry pointed out to me that our braced-init-list as a template-argument
extension doesn't work as expected when we aggregate-initialize.  Since
aggregate list-initialization is a user-defined conversion sequence, we
allow it as part of a converted constant expression.

Co-authored-by: Jason Merrill <jason@redhat.com>
gcc/cp/ChangeLog:

	PR c++/95369
	* call.c (build_converted_constant_expr_internal): Allow
	list-initialization.

gcc/testsuite/ChangeLog:

	PR c++/95369
	* g++.dg/cpp2a/nontype-class38.C: New test.